This invention relates to a method of and apparatus for observing the arrangement of atoms on a surface, in which an electromagnetic wave produced from the surface of a sample by irradiating an electron beam on the surface of the sample is detected utilizing the total reflection of the electromagnetic wave on the surface of the sample, the scanned image being displayed.
It is known that a superlattice structure which is peculiar to the surface of a crystal and completely different from that in the interior of a crystal is formed on the clean surface of the crystal of a semiconductor or a metal, or on such a surface with one atomic layer of metal or the like absorbed. However, the concrete arrangement or positions of atoms, namely the surface structure of atoms, has been little clarified. For example, an ultrastructure called "7.times.7 structure" which has seven times as many unit cells as the bulk structure is formed on the surface of Si(111). Great interest has been aroused in the arrangement of the atoms in the 7.times.7 structure, and study on it has been carried out by various means. Among these, a typical means for observing the arrangement of atoms will be described in the following.
An electron microscope, a scanning electron microscope, a field ion emission microscope, and the like are known as apparatuses with high resolution for obtaining a magnified image of a substance.
The electron microscope or the scanning microscope produces an image of the potential of an atom projected in the direction in which the electron beam scans. Therefore, if the electron beam enters in the direction of a specific axis of a crystal, many atoms overlap in this direction, thereby constituting an atom column, and the lattice image of the crystal with their potentials overlapped can be observed. In other words, this image is the image of the atoms arranged in the form of a column, not an image of the respective atom. In addition, even this lattice image cannot be obtained except when the electron beam enters in parallel to the crystallographic axis. That is, it is difficult to observe the image of the respective atom with an electron microscope or a scanning electron microscope.
On the other hand, though the image of the arrangement of the respective atom can be observed with a field ion emission microscope (F I M), the object of the observation is limited to high-melting-point metals such as W, Mo, and Ta, and it is difficult to observe the image of the arrangement of atoms in other substances. In addition the image of an individual atom cannot be obtained with an electron microscope, a scanning microscope or a field ion emission microscope.
There is a method of detecting the X-rays radiated from the vicinity of the surface of a sample by irradiating an electron beam on the surface of the sample with an electron probe X-ray micro analyzer (X M A). However, since the angle between the X-ray and the sample is ordinarily large, at least 30.degree., and hence the stereoscopic angle to be detected is also large, about 10.degree., not only the X-rays from the element on the surface of the sample but also the X-rays from the elements in the bulk are detected simultaneously. The thickness of the bulk portion to be detected at the same time with the surface reaches as deeply as several .mu.m (several tens of thousands of .ANG.) from the surface, so that the X-rays in the several atomic layers from the surface are buried in the X-rays from the bulk. This method enables an image of an individual element to be obtained, but the resolution at present is about 5,000 .ANG.. Furthermore, even if an electron beam is finely converged, it scatters in the sample and detects the X-ray at a position about several .mu.m from the surface, and high resolution is not obtained.
Auger electron spectroscopy (A E S) is a method of elemental analysis on a surface by irradiating an electron beam on the surface of a sample and utilizing the Auger effect of the electron beam. This method is said to be the most efficient in sensitivity, but, inconveniently, this is applicable only to elements of a smaller atomic number and the sensitivity remarkably decreases with respect to elements of a large atomic number.
Recently a method of using reflection high energy electron diffraction (RHEED) was reported. (Shozo Ino et al.: Japanese Journal of Applied Physics Vol. 19, No 8, 1980, pp1451-1457). This is a method of irradiating a high energy electron beam on the surface of a sample and detecting the specific X-ray produced from the element on the surface of the sample. The X-ray take-off angle is 3.degree. to 5.degree., and the intensity of the specific X-ray is inferior in comparison with that in Auger electron spectroscopy.
The inventors of the present invention proposed a method of analyzing an element on a surface by utilizing a method of detecting an electromagnetic wave at the total reflection angle thereof. This is a method of detecting the electromagnetic wave radiated from the vicinity of a surface by excitation of an electron beam by utilizing the total reflection of the electromagnetic wave. By this method the electromagnetic wave radiated from an atom on the outermost surface can be detected with great sensitivity. Accordingly, in analyzing an element, analysis results of high sensitivity are obtained in relation to the outermost surface.
An Auger electron microscope obtains a scanned image on the basis of the above-described principle on elemental analysis, but at present the resolution is not so good. That is, since low energy Auger electrons which are emitted from the depth of a sample cannot reach the surface, those emitted from the vicinity of the surface are chiefly detected. However, the size of an Auger electron signal is ordinarily smaller than that of the background consisting of the secondary electrons, and S/N (the ratio of signal and noise) is low, so that the resolution of the Auger electron microscope which uses these Auger electrons as signals is not so good, and is now about 5,000 .ANG..
As described above, when observing the arrangement of atoms on the clean surface of a semiconductor or a metal, or such surface with an atomic first layer of metal absorbed thereon, the conventional methods cannot bring about satisfactory resolution for the following various reasons: these elements are ordinarily trace elements; since a measuring system gathers information about the elements which constitute the bulk, it is difficult to obtain the information about only the elements in the vicinity of the surface separated from the other elements with an electron microscope or a scanning electron microscope; and the S/N of the Auger electron microscope is low. In addition, the field ion emission microscope is defective in that it can observe the surface of only specific metals.